Studies of the interaction of myxoviruses (group C influenza virus in particular paramyxoviruses and pseudomyxoviruses with erythrocytes (hemagglutination, hemolysis, hemadsorption) will continue based on 1.) the further chemical and immunochemical characterization of membrane proteins, particularly native and degraded glycoproteins and glycolipids (virus receptor substance, VRS) from human, rhesus and murine erythrocytes; and 2.) the isolation of paramyxovirus (Sendai) envelope subunits and the development of corresponding monospecific antisera. A similar immunochemical approach will be directed to the study of cell fusion and polykaryon formation in cultured cells by exogenously applied inactive Sendai virus; and to the morphogenesis of viruses which fuse cells during growth in cell monolayers. In the latter category, respiratory syncytial (RS) virus and pneumonia virus of mice (PVM) will be examined also by light and electron microscopy with labeled antibody (fluorescein, ferritin, peroxidase). PVM reacts with murine erythrocytes, RS virus with none. We have murine erythrocyte components and corresponding antisera with which to examine the PVM receptor system which differs from that of myxoviruses. Certain blood group antigens, which occur in the erythrocyte glycoproteins and glycolipids mentioned above, will be examined, including the I antigen which is involved in the reactivity of cold agglutinins (for human O cells) evoked by Mycoplasma pneumoniae.